The invention concerns a piston of finest grain carbon and a method for producing a piston blank and a polyaromatic mesophase powder for producing a piston.
Special aluminum alloys have been used up to now as material for pistons in combustion engines. Disadvantageously, they have relatively large specific mass, require high production accuracy, and nevertheless exhibit high frictional loss.
Carbon pistons have been proposed to obtain pistons having a small specific mass for facilitating mass compensation and reducing the frictional loss. Moreover, exhaust gas contaminants should be reduced.
The development of carbon materials for such combustion engines demanded not only improved mechanical properties but also special thermo-physical properties, in particular high heat conductivity. The reason for the latter requirement is that combustion processes in the cylinder can result in so-called xe2x80x9cknockingxe2x80x9d in consequence of overheating. Pistons of aluminum alloys have heat conductivities of approximately 140 to 160 W/mK. The development of carbon materials to replace pistons made from aluminum alloys necessarily requires a heat conductivity of at least 60 W/mK. The minimum requirements with respect to bending strength are values of more than 120 MPa in connection with a Weibull parameter of more than 20.
As is known from the technology of producing carbon or graphite materials, the demands for a higher bending strength contradict those for high thermal conductivity. The latter is achieved only through high temperature treatment at temperatures in excess of 2500xc2x0 C. With such high temperatures, re-crystallization of the graphite matrix considerably impairs the mechanical properties, such as the bending strength.
Carbon or graphite materials have been produced by mixing, compacting and subsequently carbonizing grained carbon materials (such as coke, carbon black or graphite) with a binding agent, usually a thermoplastic resin. To obtain graphitic material, high temperature treatment up to a temperature range of more than 2500xc2x0 C. follows. This has the above-mentioned disadvantages. DE 30 34 359 C2 proposes the production of carbon materials by pulverizing coke, forming with the addition of binder resin, baking of the shaped body in a first baking stage at 450-700xc2x0 C., re-impregnating the baked material with resin following an obligatory previous cooling, and subsequently baking the impregnated material in a second baking stage at at least 1000xc2x0 C. for carbonization, wherein a graphitizing step may follow at a temperature of up to 3000xc2x0 C.
DE 196 28 965 C2 discloses a method for the manufacture of a hollow tubular body made from carbon having high density, high strength and high heat conductivity. A green body is pressed, carbonized and subsequently graphitized. A self-blocking fine carbon powder, without binder, (preferably a carbon mesophase) having a powder density in accordance with DIN 51 913 of about 1 g/cm3 and with an average grain size between 5 and 20 hydrometer is thereby pre-compressed. The pre-compressed powder is pressed under a pressure of between 50 and 150 MPa about a rigid plunger to generate the hollow green body. The pressure is subsequently reduced steadily at a rate between 0.19 and 6 MPa/Min. For carbonization, the green body is then initially heated in an environment at a rate of 25 K/Min. to a temperature of 200xc2x0 C. and subsequently up to a holding temperature between 500 and 700xc2x0 C. at an extremely slow heating speed between 0.05 to 0.5 K/Min. The holding temperature is maintained for a certain holding time. The temperature is then increased to a carbonizing temperature between 800 and 1200xc2x0 C. at a rate of 0.05 to 1 K/Min. and likewise held at this temperature. The carbon body is subsequently heated to a graphitizing temperature between 2000 and 3000xc2x0 C. in an inert atmosphere. Although the procedure may be useful for its intended purpose of producing head-shaped hollow bodies, it is not applicable to pistons of motor vehicles. In particular the publication provides only little information concerning strength values resulting from the procedure. No bending values are indicated. The results may be adequate for a container, but certainly not for a motor vehicle piston.
The known method is extremely demanding due to the mixing requirement and, in particular, due to the impregnation with resin, since this requires intermediate cooling after the first baking stage. The extremely long impregnation and heating up times are also inefficient, the latter taking up to several days. The carbon material obtained is not intended for pistons and is not suitable therefor, since the bending strength is far below that which is required.
DE 44 37 558 A1 also describes production of graphite by mixing coke with a resin binder. The disclosed process is also extremely demanding. The coke powder used has an average particle size of 1 xcexcm (which is actually irrelevant from a technical point of view). Mixing with resin must be effected through kneading under increased pressure and the mixture must be cooled down and re-powdered to an average particle size of 4 xcexcm. Compacting of this powder is not possible at an earlier stage. Due to the high final treatment temperature of 2800xc2x0 C., this material most certainly has a high heat conductivity on the order of 60 W/mK, although this is not stated.
Furthermore, the use of carbon fiber reinforced carbon (CFC) has been proposed (e.g. WO97/32814 A1). Such materials are extremely expensive due to the carbon fibers and also due to the high production costs per se, as the matrix is usually formed by inside pore separation from the gaseous phase. These materials are thus not suited for an economical production of pistons which can compete with aluminum pistons. The behavior during use is also not known.
The production of carbon materials on the basis of polyaromatic mesophase has also been suggested.
Wolf, R. et al, xe2x80x9cDevelopment of Binderless Carbon-Mesophase for Production of High Strength Graphitesxe2x80x9d (Mater.; Funct.Des.; Proc. Eur. Conf. Adv. Processes Appl., 5th (1997), volume 2, 2/341-2/344. Editor(s): Sarton, L. A.; Zeedijk, H. B., Publisher: Netherlands Society for Materials Science, Zwijndrecht, Netherlands) describe carbon materials having a bending strength between 75 and 125 MPa and thermal conductivities of 45 to 60 W/mK as well as 15 W/mK. The materials, having a thermal conductivity of 45 and 60 W/mK, are proposed for use as pistons in combustion engines. They have, however, low bending elongation of e.g. 0.625%, (estimated from the bending strength and the elasticity modulus using Hooke""s Law).
Mxc3x6rgentaler, K. D. xe2x80x9cDie Entwicklung einer Technologie fxc3xcr die konturnahe Herstellung von Kolben fxc3xcr Verbrennungsmotoren aus hochfesten Feinstkornkohlenstoffenxe2x80x9d (The Development of a Technology for Close-Tolerance Production of Pistons for Combustion Engines of Highly Rigid Finest Grain Carbon) (Werst. Verkehrstech., Editor(s): U. Koch, Publisher: DGM Informationsgesellschaft, Oberursel Symp. 2, Werkstoffwoche ""96 (1997) Meeting Date 1996, 67-72) discloses a raw material named CARBOSINT which allegedly has properties similar to those of a polyaromatic mesophase powder. This raw material leads, however, to extremely hard and brittle carbon material. The extensive hardness requires extremely demanding processing such that these carbons are poorly suited for the mass production of pistons which are in any event, unacceptably brittle. CARBOSINT is intended to have a portion of toluene-insoluble components (TI) of 97% and a portion of quinoline-insoluble components (QI) of 57%. Thus, the difference between the toluene-insoluble components and the quinoline-insoluble components is 40%. After sintering, small bodies show a strength between 181 and 197 MPa in the 3 point bending strength test. Isostatically pressed large bodies having a size up to 90xc3x9790xc3x97110 mm show a strength in the 3 point bending strength test of 148 to 152 MPa after graphitization, wherein processing times of 3 months are required (Chemische Rundschau, Volume 46, Edition 13, page 3; Carbon for Pistonsxe2x80x94New Material for Combustion Engines). These results are completely impractical from a technological and economical viewpoint.
According to Wolf, R., xe2x80x9cDetermination of Suitable Mesophase Powders as Raw Materials for the Industrial Processxe2x80x9d (Extended Abstracts, International Carbon Conference, Essen, June 1992, pages 964-966) materials produced through sintering of polyaromatic mesophase powder have bending strengths in the 3 point bending strength test of between 90 or 120 MPa and heat conductivities of 60 or 50 W/mK. The latter case is a boron-containing material having a boron content of 10%. The bending elongation of the materials is only between 0.69 and 0.67%. In all proposals, the piston must be produced from solid material which is not economically competitive with aluminum pistons due to the unacceptably large degree of effort.
Hxc3xcttner, W. et al., xe2x80x9cEntwicklung von Kolben aus Feinkornkohlenstoffxe2x80x9d, in Erdxc3x6l, Erdgas, Kohle (Development of Pistons from Fine Grain Carbon in Crude Oil, Natural Gas, Coal), volume 2, February 1991, pages 81 ff also discusses fine grain carbon on the basis of mesophase intended to have, without graphitization, a heat conductivity of 45 W/mK at a bending strength of 140 MPa. Carbons without high temperature/graphitization treatment are highly susceptible to oxidation and are thus not suitable as material for pistons.
It is the underlying purpose of the invention to propose a piston for a combustion engine, a method for its production and a suitable starting material which eliminate the above-mentioned disadvantages.
In accordance with the invention, this object is achieved with a piston of finest grain carbon having a bending elongation of more than 0.8%, an average interlayer distance c/2 of less than 0.35 nm, an average crystallite size in the c direction of more than 5 nm and a heat conductivity of at least 10 W/mK, wherein the piston is preferably derived from a close tolerance quasi-statically compacted piston green product of polyaromatic mesophase which was subjected to high temperature treatment.
To achieve the above-mentioned object, a method for producing a piston blank of finest grain carbon is also proposed comprising the following steps:
a) compacting a polyaromatic mesophase powder into a piston green product, nearly having its final shape, with a portion of quinoline-insoluble components of xe2x89xa785 weight %, preferably xe2x89xa788 weight %, and with a portion of toluene-insoluble components of xe2x89xa790 weight %, preferably xe2x89xa793 weight %, wherein a shaped form produced from the powder after sintering at ambient pressure in a non-oxidizing atmosphere up to 1000xc2x0 C. has a mass residue of more than 90 weight % of the mass before sintering;
b) heating the green product at ambient pressure in a non-oxidizing atmosphere to a temperature between 900 and 1300xc2x0 C. and maintaining it at this temperature (sintering);
c) carrying out a high temperature treatment of the shaped form generated according to b) while heating to a temperature between 1400 and 2400xc2x0 C. and maintaining this temperature for 2 to 20 hours (graphitizing); and
d) cooling the shaped form to ambient temperature with a cooling speed of less than 4 K/min.
To solve the above mentioned object, the invention furthermore provides a polyaromatic mesophase powder for producing a piston blank which is characterized in that
a) it has a proportion of quinoline-insoluble components of xe2x89xa785 weight %, preferably xe2x89xa788 weight %;
b) a proportion of toluene-insoluble components of xe2x89xa790 weight %, preferably of xe2x89xa793 weight % and is further characterized in that
c) a shaped form produced from the powder after sintering in non-oxidizing atmosphere at a ambient pressure of up to 1000xc2x0 C. has a mass residue of more than 90 weight % of the mass before sintering.
The invention proposes a piston which has, in addition to the further parameters mentioned in the independent article claim, in particular, high bending elongation of more than 0.8%. A piston of this type has high stability during permanent operation and permits practical use of an economically justifiable piston, produced to nearly have its final shape, for permanent and wide-spread use in combustion engines, combustion machines or reciprocating compressors. The production of such a piston is facilitated, in particular, by the claimed powder and in accordance with the features of the method claims.
The inventive polyaromatic mesophase powder for production of a piston blank looses little mass during sintering. This is important for economic production, since low mass loss permits high speed during sintering which is again a precondition for shaping of the green product to its nearly-final shape. The powder furthermore displays high sintering activity, wherein after sintering, the relevant mechanical properties such as high bending strength and, in particular, excellent bending elongation are obtained. The powder moreover has high crystalline pre-arrangement, wherein the heat conductivity is high, even with high-temperature treatment at relatively low temperatures. This criterion has double importance. Use of the inventive mesophasic powder compared, in particular, to resin-mixed impregnated carbon, is considerably advantageous since after a first baking step or pre-sintering, cooling down is not required for impregnation, rather the shaped form can be further heated up to the sintering temperature after briefly staying at the pre-sintering temperature.
Limitation of the final treatment temperature substantially increases the economical efficiency of the method. Moreover, there is no significant loss in bending strength and bending elongation. The inventive procedure permits a mass production of a carbon piston which can compete with aluminum pistons. Moreover, the temperature treatment, in particular the high temperature treatment for graphitizing, can be effected at relatively low temperatures with a relatively high rate of temperature increase to help reduce production times and thus decrease costs.
In a further preferred embodiment of the invention, the piston has a heat conductivity of more than 20 W/mK, less than 60 W/mK, and preferably less than 45 W/mK. Surprisingly, precisely a piston having a relatively low heat conductivity of less than 60 W/mK or less than 45 W/mK, does not at all display, during permanent use, the disadvantages feared by experts, rather performs perfectly over the long term, as long as the other limitations recited in the independent article claim are satisfied.
In a further preferred embodiment, the piston has a bending strength of at least 120 MPa, preferably more than 140 MPa and a bending elongation of more than 0.9%, preferably more than 1,0%.
In an additional preferred embodiment, the average crystallite size in the c direction is more than 10 nm, preferably more than 15 nm. In a further development, the piston may comprise a proportion, of less than 0.15 weight %, preferably less than 0.1 weight %, of elements catalyzing the oxidation of carbon under operational conditions of the piston, in particular from the group of the transition metals, alkaline and alkaline earth metals. Moreover, the piston may comprise a proportion of less than 2 weight % of elements inhibiting oxidation of carbon under operational conditions of the piston such as boron, silicon and phosphor. The volume weight of the piston is advantageously more than 1.75 g/ccm, preferably more than 1.80 g/ccm.
In a preferred embodiment of the inventive method, a polyaromatic mesophase powder having the above-mentioned properties is used. Pressure treatment of the polyaromatic mesophase powder is thereby preferably carried out at a pressure of at least 80 MPa acting on a green product density of more than 1.25 g/ccm.
During temperature treatment, the green product is usually heated to sintering temperature and maintained at this temperature for a maximum of 120 hours, preferably not more than 50 hours.
In a preferred embodiment, the green product is initially heated up to an intermediate temperature between 350 and 450xc2x0 C. and held at this temperature for a duration of between 1.5 and 5 hours. Heating to the intermediate temperature of 350 to 450xc2x0 C. is usually effected in a time period between 4 to 40 hours, preferably within a maximum of 20 hours.
Further heating of the green product from the above mentioned intermediate temperature to the sintering temperature may be effected within 10 to 90 hours, preferably within a maximum of 40 hours.
In a preferred embodiment, the shaped product, heated to sintering temperature, is held at this temperature for up to 10 hours.
The compacted green product is thereby preferably heated with a heating speed of 0.1 to 2 K/min. The heating-up and dwell times are considerably less than those of prior art. The invention operates with relatively high heating-up speeds.
In further substantial teaching of the invention, the temperature increase is preferably not effected with constant, rather with differing heating up speeds. The pre-sintered green product is thereby preferably heated to the sintering temperature between 900 and 1300xc2x0 C. in different temperature regions with different speeds which have a mutual relationship of 1:5 to 1:1, wherein the first temperature range extends to 600xc2x0 C. and the second temperature range extends to the final sintering temperature.
A preferred embodiment of the invention provides that the sintered shaped form is initially cooled and subsequently graphitized. Towards this end, the shaped form is preferably heated to the graphitizing temperature at a heating speed between 0.1 to 2 K/min.
The blank, which has cooled down after graphitizing, can advantageously be subjected to fine mechanical or fine chemical post-processing to give the piston its final shape and/or for adjusting its properties. The final shaping thereby requires only little post-processing since, due to close-tolerance shaping of the green product, the cooled down blank nearly has its final shape. Post-treatment of the inner hollow space of the blank is, in particular, not required.
A preferred powder in accordance with the invention is characterized by
a) a pycnometer density of  greater than 1.40 g/ccm;
b) an oxygen content of less than 3 weight %, preferably less than 2 weight %;
c) an incineration residue of less than 0.25 weight %, preferably less than 0.2 weight %;
d) an average grain diameter of the grain distribution cumulative curve d50 between 3 and 12 xcexcm, preferably between 5 and 10 xcexcm
e) a coarse grain proportion of the grain distribution cumulative curve having a grain diameter of d xe2x89xa720 xcexcm of less than 5%.
The invention also concerns use of a piston made from finest grain carbon in combustion engines, in particular gasoline, diesel or gas-operated engines, and in reciprocating compressors.
Further preferred embodiments and features of the invention can be extracted from the claims and the following description of preferred embodiments of the invention with reference to the accompanying tables and diagrams.